Polyanhydroaspartic acid (also known as polysuccinimide) is the key commercial precursor to polyaspartic acid, an increasingly important biodegradable water soluble polymer. The chemistry and process fundamentals of polysuccinimide by the thermal condensation polymerization of aspartic acid are well established in the patent and journal literature. Illustrations are described in U.S. Pat. No. 5,057,597, to Koskan and U.S. Pat. No. 5,315,010 and No. 5,221,733 to Koskan et al. The weight average molecular weights obtained by thermal condensation of aspartic acid by these methods are usually about 5000.
No commercial processes employing phosphoric acid based additives to prepare relatively low weight average molecular weight polysuccinimide exist for several reasons. First, the cost of the reactants, phosphoric acid and its anhydride, are relatively high. Second, waste streams containing phosphate salts are damaging to the environment because phosphorous promotes algae blooms in lakes and rivers which in turn deplete the level of oxygen necessary to sustain marine life.
Commonly owned U.S. Pat. No. 5,508,434 to Batzel et al. describes an unstirrable, highly viscous, thin layer melt process for the production of polysuccinimide with the concomitant release of water vapor. However, this particular process requires post-synthetic treatment of the product which adds to the overall cost of production. The post-synthetic treatment includes the additional steps of collecting and crushing the porous solid succinimide then extracting the sulfuric acid and salts contained within by numerous washing steps followed by drying of the product.
The present invention provides an economical, ecologically tolerable, and phosphoric acid free solvent system manufacturing process for preparing relatively low weight average molecular weight polysuccinimide in the range of about 1000 to 5000 Mw.